Chemical Treatment of Irrigation Water for SDI System Maintenance

Dorota Z. Haman

Department of Biological and Agricultural Engineering

University of Florida

P.O. Box 110570

Gainesville, FL 32611

, Gary A. Clark,

Department of Biological and Agricultural Engineering

Kansas State University

147 Seaton Hall

Manhattan, KS 66506

Brian J. Boman,

Biological and Agricultural Engineering Department

University of Florida

Indian River Research and Education Center

2199 S. Rock Rd

Fort Pierce, FL 34945

Michael D. Dukes

Department of Agricultural and Biological Engineering

University of Florida

P.O. Box 110570

Gainesville, FL 32611

Chemical treatment of irrigation water is often required to prevent emitter plugging due to microbial growth and/or mineral precipitation. Microbial activity can generally be controlled with chlorine, while acid injection can remove scale deposits, reduce or eliminate mineral precipitation, and create an environment unsuitable for microbial growth. In addition, insecticides may be necessary to control damage due to ants and other insects.

Chlorine Injection

Chlorine is used in many municipal and industrial water supply systems and home swimming pools to prevent algae and other microorganisms from growing. Chlorine is also used for cleaning and maintaining irrigation systems. Proper injection methods, amounts, and concentrations of chemicals must be used to provide an effective water treatment program without damaging the irrigation system or the irrigated crop. Because chlorine can react with some metals and plastics, the manufacturer of irrigation system components should be consulted to make sure that problems do not occur when chlorine is used in water treatment programs.

Irrigation systems can become partially or completely clogged from biological growths of bacteria, fungi, or algae, which are often present in surface water and some ground water sources. These microorganisms use chemical elements in the water such as nitrogen, phosphorus, sulfur, or iron as nutrient sources to grow and develop. Generally, filtration alone cannot effectively remove these microorganisms. However, chlorination can be used to minimize or eliminate their growth within the pipes and other components of irrigation systems.

Chlorine is available in gas, liquid, and solid (granular or tablet) forms. However, only the liquid form (liquid sodium hypochlorite) has an Environmental Protection Agency (EPA) special local need (SLN) label for use as a pesticide in irrigation systems in some parts of the United States. Local laws and regulations should be consulted prior to adoption of any chemical treatment practice.

These three different chlorine forms react differently with the irrigation water, depending on the other chemicals or elements in the water. In addition, chlorine may cause changes in the pH of the water, or precipitate some other element which could result in clogging of the microirrigation components.

Chlorine gas (Cl2) is commonly used in municipal water treatment systems and is a dangerous source of chlorine. As chlorine gas reacts with water, hypochlorous acid (HOCl), hydrogen (H+), and chloride (Cl-) are formed. This reaction lowers the pH of the irrigation water. The level of the change in pH depends on how much chlorine gas is injected and on the buffering capacity of the water.

Granular (powered or tablet) forms of chlorine are commonly used to chlorinate swimming pools. Calcium hypochlorite found at local swimming pool supply stores is the form that is typically used. Dissolving calcium hypochlorite in water will result in the formation of hypochlorous acid (HOCl) and hydroxyl ions (OH-), a reaction that raises the pH of the water. Calcium hypochlorite may also react with other elements in irrigation water to form precipitates, which could clog microirrigation emitters and thus defeat the purpose for chlorination. As a result, liquid chlorine (sodium hypochlorite) rather than solid calcium hypochlorite is recommended for use in irrigation systems, especially when the water source is high in other minerals.

Liquid sodium hypochlorite (laundry bleach) is most commonly found and used as laundry bleach. Mixing liquid sodium hypochlorite in water results in the formation of hypochlorous acid (HOCl) and hydroxyl ions (OH--1), a reaction that also raises the pH of the water. Unlike the calcium added in the solid chlorine form, the sodium added in this liquid form does not contribute to clogging problems. Neither the sodium nor the chlorine added to the water would be detrimental to crops or soils at the low concentrations typically used to treat irrigation systems.

Some problems can occur with water sources that have high iron levels. Hypochlorous acid reacts with iron in solution and oxidizes ferrous iron to the ferric form. The ferric iron then becomes the insoluble ferric hydroxide as a granular precipitate. Chlorine should be injected before (upstream from) the filters so that these precipitates may be trapped in the filters. Chlorine also reacts with hydrogen sulfide and forms elemental sulfur. Because some of the chlorine reacts with the sulfide or ferrous ions, additional chlorine must be provided for these reactions to occur in addition to the chlorine that is needed for the control of microorganisms.

Most microorganisms are controlled when the free residual chlorine concentration is 1 ppm or greater at the most distant end of the irrigation system. However, higher concentrations must be injected due to the inherent chlorine demand of the constituents associated with different water sources. A chemical water test can be used to determine the levels of hydrogen sulfide or ferrous iron present in solution. Based on this test the rate of chlorine injection can be calculated. As a start, use 2 ppm of chlorine for each ppm of hydrogen sulfide, plus 0.6 ppm of chlorine for each ppm of ferrous iron. Water from surface sources such as lakes, ponds, or canals should be initially treated with approximately 5 to 10 ppm of chlorine. Higher chlorine levels may be needed to treat water with high amounts of microbial activity that as may occur during the warmer months of the year.

Chlorine injection rates should be checked by testing the treated water at the most distant part of the irrigation system using a test kit designed to measure "free" residual chlorine. Residual chlorine concentrations of 1 to 2 ppm at this location indicate that active chlorine still exists after the water and system parts have been appropriately treated.

The amount of active chlorine can be tested using a color indicating test kit that measures "free" residual chlorine. A test kit that only measures total chlorine should not be used. While levels of total chlorine may appear to be adequate, the active "free" residual form may not be adequate for a complete treatment of the water. Chlorine test kits/DPD kits can be purchased from either swimming pool or irrigation supply companies.

After determining the desired chlorine concentration, the proper amount to be injected must be determined. The amount of chlorine to apply to the irrigation system will depend on the desired chlorine concentration in the irrigation water, the concentration or strength of the liquid chlorine source, and the flow rate of water through the irrigation system.

The rate of injection of any liquid chemical is directly proportional to the flow rate of water in the irrigation system and can be calculated from the following equation:

Qchemical = K (u*Qwater/ C) (1)

Where :Q chemical is the chemical injection rate (L/hr or gph), Qwater is the water flow rate in the irrigation system (L/sor gpm), K is a conversion constant ( K= 3.60 x 10-3 for SI units and K= 5.01 x 10-4 for English units), u is the desired concentration of chlorine in the irrigation water (ppm), and C is the concentration of the component in liquid to be injected (kg/L or lb/gal).

Liquid sodium hypochlorite is the most convenient and generally safest form of chlorine available to inject into irrigation systems. Stock solutions can be purchased with concentrations of 5.25, 10, or 15 percent available chlorine. Table 1 or Equations 2 – 4 may be used to determine the chlorine solution injection rate in gallons per hour (gph) of liquid chlorine for different desired ppm injection levels and irrigation system flow rates. Equations 2 - 4 are specific for liquid chlorine injection and the listed stock solution chlorine concentrations.

For a 5.25% available chlorine stock solution:

Injection Rate5.25, gph = (Desired Concentration of Cl in Irrigation Water, (ppm) (Irrigation Flow Rate, gpm)/ 971 (2)

For a 10% available chlorine stock solution:

Injection Rate10, gph = (Desired Concentration of Cl in Irrigation Water, ppm) (Irrigation Flow Rate, gpm)/ (ppm)(Irrigation Flow Rate, gpm)/1850 (3)

For a 15% available chlorine stock solution:

Injection Rate15, gph = (Desired Concentration of Cl in Irrigation Water, ppm) ppm)(Irrigation Flow Rate, gpm)/2775 (4)

For example, an irrigation system has a flow rate of 500 gpm and the water is to be treated withto 8 ppm of available chlorine using a stock solution with 10% available chlorine. Using Equation 3, the injection rate of the stock solution into the irrigation system should be:

(8 ppm)(500 gpm)/1850 = 2.2 gph

Similarly, from Table 1, for a treatment level of 8 ppm and a 10% available chlorine concentration, read an injection rate of 0.4 gph. Note that this is the required injection rate for each 100 gpm. Thus, for 500 gpm, the injection rate would be five times as large, or 2.0 gph.

If the stock solution concentration was 5.25% available chlorine, then the injection rate should be:

(8 ppm)(500 gpm)/971 = 4.1 gph

From Table 1, for a treatment level of 8 ppm and a 5% available chlorine concentration, read an injection rate of 0.9 gph per 100 gpm of irrigation flow rate. Then for 500 gpm, the injection rate would be five times as large, or 4.5 gph.

If the calculated injection rate is too small for the injection pump, the chlorine stock solution can be diluted with irrigation water. Thus, if the 10% stock solution is diluted with 1 part water and 1 part 10% chlorine solution, the new stock solution would be diluted by 1/2. It would then have 5% available chlorine, assuming that the water added did not tie up any of the available chlorine. Likewise, if the 10% stock solution is diluted with 4 parts water and 1 part 10% chlorine solution, the new stock solution would be diluted by 1/5, and it would have 2% available chlorine.

Chlorine is a powerful oxidizing agent and it must be handled carefully. A fresh water source should be available at the field site where liquid sodium hypochlorite is being used so that any contact or spills can immediately be washed off. Protective clothing should be worn while handling this chemical and the associated injection equipment. Goggles should be worn to protect eyes against splashes and gloves should be used to protect hands.

Chlorine gas is a respiratory irritant, which affects the mucous membranes. It can be fatal after a few breaths at 1000 ppm. Therefore, users of chlorine gas must exercise extreme caution to ensure that it is safely injected. Maximum air concentrations should not exceed 1 ppm for prolonged exposure. Chlorine gas should only be used in well-ventilated areas so that any leaking gas cannot concentrate. While this form of chlorine is commonly used in municipal water treatment systems, it should only be used by experienced and/or licensed users. For safety, only vacuum type injectors should be used.

Table 1. Liquid chlorine (sodium hypochlorite) injection rates in gallons per hour (gph) per 100 gallons per minute (gpm) of irrigation water flow rate for different levels of stock solution concentrations of available chlorine (%) and the desired chlorine
concentration (ppm). concentration (ppm).
Treatment Level
(ppm) / Concentration of available chlorine in stock solution*
(percent)
1 / 2 / 3 / 4 / 5 / 10 / 15
(gph of injection per 100 gpm of irrigation flow rate)
2 / 1.1 / 0.6 / 0.4 / 0.3 / 0.2 / 0.14 / 0.09
4 / 2.2 / 1.1 / 0.7 / 0.6 / 0.4 / 0.22 / 0.15
6 / 3.3 / 1.7 / 1.1 / 0.8 / 0.7 / 0.3 / 0.2
8 / 4.4 / 2.2 / 0.5 / 1.1 / 0.9 / 0.4 / 0.3
10 / 5.5 / 2.8 / 1.8 / 1.4 / 1.1 / 0.6 / 0.4
15 / 8.3 / 4.1 / 2.8 / 2.1 / 1.6 / 0.8 / 0.5
20 / 11.0 / 5.5 / 3.7 / 2.8 / 2.2 / 1.1 / 0.7
25 / 13.8 / 6.9 / 4.6 / 3.4 / 2.8 / 1.4 / 0.9
30 / 16.5 / 8.3 / 5.5 / 4.1 / 3.3 / 1.6 / 1.1
40 / 22.0 / 11.0 / 7.3 / 5.5 / 4.4 / 2.2 / 1.5
50 / 27.5 / 13.8 / 9.2 / 6.9 / 5.5 / 2.8 / 1.8
75 / 20.6 / 13.8 / 10.3 / 8.3 / 4.1 / 2.8
100 / 27.5 / 18.3 / 13.8 / 11.0 / 5.5 / 3.7
150 / 27.5 / 20.6 / 16.5 / 8.3 / 5.5
200 / 27.5 / 22.0 / 11.0 / 7.3
* These are commercially available concentrations. Other concentrations are obtained by diluting with water.

Summary:

1.  Have your irrigation water tested.

2.  Select the source of chlorine.

3.  Determine how much chlorine should be injected to obtain the desired concentration (ppm) of chlorine in the irrigation water (Eqs. 2-4 or Table 1).

4.  Inject the calculated amount of chlorine solution.

5.  Measure the concentration of “free” chlorine at the end of the last lateral.

6.  If the amount is less than 2 ppm increase the chlorine injection rate.

Acid Injection

Hypochlorous acid (HOCl) is the effective agent that controls bacterial growths. The amount of HOCl that will be present in solution, and thus active, will be greater at lower pH levels (more acidic conditions). At a water pH of 8, only about 22% of the chlorine injected will be in the active HOCl form, at a pH of 7, about 73% will be in the HOCl form, and at a pH of 6, about 96% will be in the HOCl form (Nakayama and Bucks, 1986). Thus, if the irrigation water pH is high, the effectiveness of the injected chlorine may be enhanced by injecting an acid to reduce the pH of the water before injecting chlorine. In addition to increasing the effectiveness of chlorine, acid injection can also prevent the precipitation of minerals, which may plug microirrigation systems. However, it is normally only necessary to reduce the pH one or two units to achieve these desirable benefits.